Conventionally, in order to reduce the size of an optical pickup, due to the necessity to avoid problems of heat emission and unnecessary radiation of electromagnetic waves from a semiconductor laser which is a light source and from a peripheral circuit thereof, the semiconductor laser and the optical head are disposed separately.
As an optical wiring to separate, as described above, an optical head and a semiconductor laser and to lead a light beam from the semiconductor laser into the optical head, an optical fiber is efficient and utilized.
Recently, attempts have been made to improve coupling efficiency between a semiconductor laser and an optical fiber, and to permit less alignment accuracy at a coupling portion.
For example, there was a method in which the diameter of the core of an optical fiber was enlarged at one end to which a semiconductor laser was coupled, and the core diameter was gradually reduced toward the other end, so as to come to its original core diameter of the optical fiber. By using this method, less alignment accuracy between the semiconductor laser and the optical fiber can be permitted,—and therefore the alignment was made without difficulties.
However, because in this method cross section of the core is circular at the end of the optical fiber to which the semiconductor laser is coupled, in order to improve the coupling efficiency from the semiconductor laser to the optical fiber this method requires an optical system such as an anamorphic prism which converts a laser light beam of elliptical cross section emitted from the semiconductor laser into that of circular cross section.
When such optical system as an anamorphic prism is constructed, it becomes difficult to reduce the size of an optical pickup.
In addition, since a conventional optical fiber is of a polarization-independent type, it is difficult to improve efficiency in utilizing a light beam, as an optical pickup.
In order to solve the above problems an optical fiber of such structure has been proposed, in which coupling efficiency is improved by making cross section of a laser light beam elliptical at an end of an optical fiber so as to correspond to that of a laser light beam emitted from a semiconductor laser, and conversion from a light beam having an elliptical cross section into a single-mode light beam having a circular cross section is made possible.
Specifically, an optical fiber formed by covering the core thereof with cladding is composed of: a first optical fiber in which an optical axis is assumed to be Z, both of the refractive index distribution profiles of the core in X and Y directions among three axes of X, Y and Z perpendicular to each other are made to quadratic index distribution profiles, and slopes of the refractive index distribution profiles in the corresponding X and Y directions are made different values from each other; and a second optical fiber formed of a single-mode optical fiber the core of which is covered with cladding, attached to one end of the first optical fiber.
However, even in the above optical fiber it is still difficult to sufficiently improve efficiency in utilizing a light beam as an optical pickup, because a single-mode optical fiber constituting the second optical fiber is a polarization-independent type optical fiber.
In order to solve the above described problems, the present invention provides, using an optical fiber module provided with optical fibers, an optical pickup which can be reduced in its size and also can utilize a light beam from a light source at high efficiency.